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<title>Using as: Input Files</title>

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<a name="Input-Files"></a>
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<a name="Input-Files-1"></a>
<h3 class="section">1.5 Input Files</h3>

<a name="index-input"></a>
<a name="index-source-program"></a>
<a name="index-files_002c-input"></a>
<p>We use the phrase <em>source program</em>, abbreviated <em>source</em>, to
describe the program input to one run of <code>as</code>.  The program may
be in one or more files; how the source is partitioned into files
doesn&rsquo;t change the meaning of the source.
</p>
<p>The source program is a concatenation of the text in all the files, in the
order specified.
</p>
<p>Each time you run <code>as</code> it assembles exactly one source
program.  The source program is made up of one or more files.
(The standard input is also a file.)
</p>
<p>You give <code>as</code> a command line that has zero or more input file
names.  The input files are read (from left file name to right).  A
command line argument (in any position) that has no special meaning
is taken to be an input file name.
</p>
<p>If you give <code>as</code> no file names it attempts to read one input file
from the <code>as</code> standard input, which is normally your terminal.  You
may have to type <tt class="key">ctl-D</tt> to tell <code>as</code> there is no more program
to assemble.
</p>
<p>Use &lsquo;<samp>--</samp>&rsquo; if you need to explicitly name the standard input file
in your command line.
</p>
<p>If the source is empty, <code>as</code> produces a small, empty object
file.
</p>

<a name="Filenames-and-Line_002dnumbers"></a>
<h4 class="subheading">Filenames and Line-numbers</h4>

<a name="index-input-file-linenumbers"></a>
<a name="index-line-numbers_002c-in-input-files"></a>
<p>There are two ways of locating a line in the input file (or files) and
either may be used in reporting error messages.  One way refers to a line
number in a physical file; the other refers to a line number in a
&ldquo;logical&rdquo; file.  See <a href="Errors.html#Errors">Error and Warning Messages</a>.
</p>
<p><em>Physical files</em> are those files named in the command line given
to <code>as</code>.
</p>
<p><em>Logical files</em> are simply names declared explicitly by assembler
directives; they bear no relation to physical files.  Logical file names help
error messages reflect the original source file, when <code>as</code> source
is itself synthesized from other files.  <code>as</code> understands the
&lsquo;<samp>#</samp>&rsquo; directives emitted by the <code>gcc</code> preprocessor.  See also
<a href="File.html#File"><code>.file</code></a>.
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